Support custom cast function

[Jonathan MASSUCHETTI]

It would like to be able to cast a value using a callable.

class Foo {
  public static function fromString($bar) {
    // some logic
  }
}

$schema = Expect::structure([
  'foo' => Expect::string()->castTo(fn ($v) => Foo::fromString($v));
  'foo2' => Expect::string()->castTo('Foo::fromString');
});

I'm willing to make a PR for this if you accept the feature to be integrated in the project.

[Jan Tvrdík]

The idea is good, but it would be better to use different method name, to more accurately describe the action, e.g. transform (that is the name used by Zod)